ADC! Seems like only yesterday.
I'm currently designing and buying two MLB's with CSP/BGA's on 30 mil
centers. The sixteen layer version, with differential broad side impedance
requirements on specified digital layers, is about 120 mils thick with 10
mil interfacial vias - currently. It costs about 1500 dollars in 10 piece
quantities for 5-7 day turn from a NATIONALLY RECOGNIZED board shop. No
quotes on production quantities yet. It will cost about 300 dollars more for
the micro via version with buried vias connecting the internally placed
dogbones allowing pad only outers.
The 22 layer version is priced at about the same ratio starting about 2000
dollars but must have the blind micro vias and buried vias as its thickness
is about 180 mils. Simply, there is no choice.
As these boards exceed my expetations for aspect ratio, partly considering
the plating thickness issue of .8 mils minimum, it will be respun to include
micro vias in the BGA pads, eliminating dogbones on the outers, and reducing
the aspect ratios in that area as the bones will be interconnected on the
inners using buried vias.
Again, the cost of the new design will be about three hundred dolars more
for the same turn times. However, it is expected to save much more as a more
reliable product. As Werner said, who has the data concerning plating
thickness reduction as it is now being practiced. However, with a Z axis
expansion rate of about 50 ppm/degree C, and a ductility factor nearing 20%,
with a 170 degree Tg material, the hole walls should stay together under
thermal stress.
We have similar designs for varying other layer counts. There seems to be
about a 20%, or so, cost differential between the two choices, but the long
term effects should offset the decision.
Earl Moon
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